1. Field of the Invention
The subject matter of this invention pertains to the preparation of mixtures of diphenylmethane diisocyanates and polymethylene polyisocyanates (hereinafter referred to as crude MDI). Crude MDI is prepared in accordance with this invention by reacting aniline and an O-alkyl carbamate, preferably in the presence of urea and an alcohol, to form N-phenylurethane. The N-phenylurethane is condensed with formaldehyde or various other compounds to form methylene bis(phenylurethane) and polymethylene polyphenylurethane (hereinafter referred to as crude MDU) which is thermally cleaved to form crude MDI.
2. Description of the Prior Art
It is known that crude MDI can be prepared by reacting diphenylmethane diamines and polyphenyl polymethylene polyamines (hereinafter referred to as crude MDA) with phosgene to form corresponding carbamic acid chlorides which can be thermally cleaved into crude MDI. This process is expensive and potentially dangerous because phosgene is toxic and it, along with the carbamic acid chlorides, produces hydrogen chloride which is highly corrosive.
The crude MDA for the process is produced by the condensation of aniline with formaldehyde in the presence of acid catalysts. See British Pat. No. 648,787 and Canadian Pat. No. 700,026. On an industrial scale, hydrogen chloride is added in large quantities. The hydrogen chloride must be neutralized and separated from the reaction mixture.
Because of the problems associated with using phosgene, efforts have been made to develop processes for producing crude MDI without using it. For example, British Pat. No. 1,025,436, German Published Application No. 1,815,517 (U.S. Pat. No. 3,576,835), and German Published Application No. 1,931,212 (U.S. Pat. Nos. 3,654,279 and 3,781,321) describe the preparation of isocyanates by the reaction of nitroaromatics and carbon monoxide. The problems with such processes are that they are technically complicated and cannot be used to produce crude MDI because there is no suitable method for preparing polynitro-polyphenylmethane.
It has been suggested that isocyanates can be prepared by thermally cleaving urethanes. In attempting to accomplish this, several methods of preparing the urethanes have been tried. According to German Pat. No. 1,042,891 (U.S. Pat. No. 2,946,768), a synthesis of crude MDU is possible by means of condensation of phenylurethane with formaldehyde. However, the described process did not result in an industrial scale success because the manufacture of phenylurethane requires the reaction of aniline with chlorocarbonates and/or phenylisocyanate with alcohol. Consequently, the use of phosgene and the problems it creates were not eliminated. Moreover, the condensation product of urethane and formaldehyde contain hydrolysis products with free amino groups and considerable amounts (15 to 50 percent) of N--C bonded components which cannot be cleaved into crude MDI.
A chlorine-free process for the manufacture of phenylurethanes by reacting aniline, urea and alcohol is described in U.S. Pat. No. 2,409,712, and U.S. Pat. No. 2,806,051. However, this process was not successful due to the relatively modest yields in comparison with the phosgenation of aniline and subsequent reaction with alcohol. Therefore, it has not been mentioned in processes for the urethane manufacture described in later publications.
A process for the manufacture of N-substituted urethanes, for instance, phenylurethane, is described in German Published Application No. 2,160,111 (U.S. Pat. No. 3,763,217). The urethanes are prepared by reacting an organic carbonate with aniline in the presence of a Lewis acid. The drawback of this process is the fact that alkylcarbonates must be produced from phosgene and alcohol, or by the technically and still problematic, catalytic cooxidation of carbon monoxide and alcohol, which is expensive. Moreover, the rate of reaction is rather slow and N-alkyl anilines are competing products.
British Pat. No. 1,247,451 describes the preparation of aryl urethanes without using phosgene by reacting nitroaromatics, alcohol, and carbon monoxide. The arylurethanes can be transformed into isocyanates with greater chances of success.
According to data in German Published Application No. 1,568,044 (U.S. Pat. No. 3,467,694), urethanes are produced by reaction of organic nitro compounds, carbon monoxide and hydroxyl-containing compounds in the presence of a catalyst, which consists of a noble metal and a Lewis acid. The reaction occurs under essentially anhydrous conditions in the absence of hydrogen, under increased pressure, and at temperatures above 150.degree. C. According to German Published Application No. 2,343,826 (U.S. Pat. No. 3,895,054), urethanes are obtained from hydroxyl group containing compounds, carbon monoxide and nitro, nitroso, azo and azoxy group containing compounds in the presence of sulfur, selinium, a sulfur and/or selinium compound, and at least one base and/or water. German Published Application No. 2,623,694 (U.S. Pat. No. 4,080,365) describes the manufacture of aromatic urethanes in the presence of selinium containing catalyst systems as well as special aromatic amino and urea compounds.
However, the aforementioned processes also have serious drawbacks. They employ toxic carbon monoxide and catalysts which are toxic or which form toxic compounds during the reaction, such as hydrogen selenide or hydrogen sulfide, or they employ catalysts which are very expensive and are difficult to recycle, such as palladium. They also require high technical expenditure and expensive safety measures.
As was previously mentioned, N-phenylurethane can be condensed with formaldehyde to form crude MDU which can be thermally cleaved to crude MDI. See German Pat. No. 1,042,891. However, according to German Published Application No. 2,832,379, a better form of crude MDU is produced if water is removed from the obtained condensate and if the product is treated with acids in order to complete the transposition of N--C-- to C--C-- bonded products. Nevertheless, the use of large acid quantities and the formation of resultant byproducts, for instance amines, create a considerable load on the waste water. Therefore, the improved product quality of the crude MDI, according to German Published Application No. 2,832,379, must be obtained by additional increased technical expenditure.
As already mentioned, N-substituted urethanes can subsequently be cleaved thermally into isocyanates. However, there are many problems involved in the cleaving process. The thermal cleaving is accompanied by various undesired secondary reactions. These include, for instance, the decarboxylation reaction of the urethanes which can be accompanied by the formation of primary and secondary amines.
Problems arise regardless of whether the cleaving is done in the vapor phase, liquid phase, or solid phase. According to German Published Application No. 1,944,719 (British Pat. No. 1,247,451), cleaving of the urethanes in the vapor phase is carried out at temperatures of 400.degree. C. to 600.degree. C. in the presence of a Lewis acid as catalyst with the isocyanate and the alcohol being separated by fractional condensation. Vapor phase, in this case, is defined in such a manner that the product mixture, possibly including the solvent, are present in the vapor phase after the cleaving regardless of whether the urethanes to be cleaved are added in the gaseous, liquid, or solid form.
Toluene diisocyanate is produced, for instance, by means of the cleaving of toluene-2,4-diethylurethane in the presence of iron-(III)-chloride. Drawbacks of this reaction include low yields combined with considerable quantities of a polymeric byproduct, and the decomposition and corrosiveness of the catalyst. German Published Application No. 2,410,505 (U.S. Pat. No. 3,870,739) describes a process by which the urethane is cleaved at a temperature of 350.degree. C. to 550.degree. C. and a pressure of less than the (m+1) times the vapor pressure of the isocyanate product in a catalyst-free pyrolysis zone within 15 seconds. Drawbacks of this process include the fact that a large amount of heat required for the endothermal cleaving must be fed to the powdery urethane within a very short period of time and that a solid polymer, which is incurred as a byproduct, separates. This makes the implementation of a continuous process difficult.
The thermal cleaving of urethanes in the liquid phase is described in German Application No. 2,421,503 (U.S. Pat. No. 3,962,302) and 2,530,001 (U.S. Pat. No. 3,919,280). According to German Application No. 2,421,503, the urethanes are dissolved in an inert solvent such as alkylbenzenes, linear and cyclic hydrocarbons, and/or phthalates, and are cleaved under normal or excess pressure at temperatures of 175.degree. C. to 350.degree. C. The resultant isocyanate and alcohol are isolated and separated with the aid of the solvent as carrier and/or by using an inert gas as a carrier. According to German Application No. 2,530,001, higher molecular, possibly substituted aliphatic, cycloaliphatic or aromatic hydrocarbons, ethers, esters, or ketones, are used as the reaction medium. For separating the cleaved products, this application only cites distillation with isocyanate, alcohol and carrier material being distilled overhead, while the reaction medium remains as bottom fraction.
Some of these drawbacks can be eliminated by using the process described in German Published Application No. 2,635,490. This patent describes a process for preparing aromatic isocyanates wherein the urethanes are brought into contact with a solution of at least one metal ion such as ions of copper, zinc, aluminum, tin, titanium, vanadium, iron, cobalt and nickel as catalysts. The catalysts are dissolved in an inert solvent having a boiling point of 200.degree. C. and are used in a concentration of at least 0.001 percent by weight relative to the solvent. The reaction occurs at temperatures of 150.degree. C. to 350.degree. C. under reduced pressure. The resultant cleaved products are separated by fractional condensation. In some cases, this method is successful in converting urethanes into isocyanates with good yields. However, it is noteworthy that the manufacture of crude MDI is not described by example in this patent. The reason is that crude MDI cannot be completely distilled with the aid of solvents as carrier materials and, therefore, can not be isolated from the catalyst, possibly unreacted raw materials, and byproducts.